The inspections are focused on plugs used to seal an area set aside for extra emergency doors that are not required on United and Alaska Max 9s.
"Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug - for example, bolts that needed additional tightening," United said.
RELATED: Federal officials order grounding of some Boeing 737 Max 9 jetliners after plane suffers a blowout
Alaska Airlines grounds Boeing 737-9 fleet after emergency landing in Portland
Boeing issued the following statement on 737-9 customer inspection findings:
"As operators conduct the required inspections, we are staying in close contact with them and will help address any and all findings. We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards. We regret the impact this has had on our customers and their passengers."
The Federal Aviation Administration grounded all Max 9s operated by Alaska and United and some flown by foreign airlines after a terrifying flight on Friday night where a piece of fuselage blew out mid-flight.
On Monday, hundreds of flights were cancelled by Alaska Airlines and United Airlines nationwide, including more than 50 cancellations at San Francisco International Airport.
Boeing 737-9 aircraft to remain grounded until 'enhanced inspections' are complete, FAA says
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The Boeing 737 MAX 9 's "door plug," which fell off after takeoff, was been found late Sunday night.
The National Transportation Safety Board says a teacher in Portland, Oregon found it in his backyard.
It left a huge hole in the fuselage, forcing an emergency landing on Friday with more than 100 passengers and crew on board.
NTSB investigators said Monday they are currently examining the door plug and will send it to the NTSB Materials Laboratory in Washington, DC for further examination.
The FAA has grounded more than 170 planes in the 737 MAX 9 fleet and that's having a big impact for travelers nationwide Monday morning.
The FAA said Monday morning the inspections needed to get certain grounded Boeing 737 MAX 9s back in the air.
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The agency saying: "Boeing 737-9 aircraft will remain grounded until operators complete enhanced inspections which include both left and right cabin door exit plugs, door components, and fasteners."
RELATED: Missing door plug that blew off Alaska Airlines flight found in Portland backyard
Missing door plug that blew off Alaska Airlines flight found in Portland teacher's backyard
Bay Area Impact
"Especially considering, since the plane is going to be a 737 MAX 9, we were concerned about it being changed," Bryce Forney, a Pleasant Hill resident said.
Forney, a Pleasant Hill resident says, he was surprised to wake up to only a one hour delay on his family's flight to Orlando, even after learning they would be boarding a 737 MAX 9.
"But I appreciate that Alaska and some other airlines took some of them out of service to do inspections," he said. "That seems like the right, cautious approach, better than waiting and finding out something might go wrong."
RELATED: 'Very frustrating': Bay Area travelers see more cancelations from Boeing 737 MAX 9 groundings
More flight cancelations in Bay Area from Boeing 737 MAX 9 groundings
Still, Alaska Airlines and United Airlines, two major operators of the Boeing 737 MAX 9, are cancelling a combined 342 flights on Monday.
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"Yesterday, I checked and I saw that my connecting flight from Houston to San Francisco was the 737 MAX and I wasn't sure whether to be concerned or not, it didn't show cancelled," Juan Carlos Guerrero, an ABC7 News producer and reporter said.
Guerrero got stuck in Houston after United abruptly cancelled his connecting flight back to SFO from Buenos Aires.
Now, with a rebooked added stop in Palm Springs, he says he's expecting to land at least eight hours later than planned.
"Of course, I'd rather be home right now but at the same time, I'd rather be on a plane that's safe. I don't want to be sucked out mid-air or anything like that so if I have to be here a little longer to make sure I get home safe, that's fine," he said.
Alaska Airlines said on Sunday, their teams are prepared and ready to perform the required inspections of the mid-exit door plugs on these planes.
United Airlines found loose bolts during preliminary inspections, company says
United Airlines said Monday afternoon that it has found loose bolts during its preliminary inspections of its 737 MAX 9 fleet. The airline did not say how many planes had loose bolts.
The company said in a statement, writing:
"Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug - for example, bolts that needed additional tightening. These findings will be remedied by our Tech Ops team to safely return the aircraft to service."
United Airlines said it has canceled 200 MAX 9 flights and expect significant cancellations on Tuesday as well.
"We have been able to operate some planned flights by switching to other aircraft types, avoiding about 30 cancellations each on Monday and Tuesday."
The Associated Press contributed to this report.